Microwave communications systems require filters with sharp frequency selectivity characteristics. These characteristics must be realized in devices of minimum weight and volume in order to be useful in microwave applications such as satellite communications. Conventional satellite communications systems employ multiplexing systems based upon wave-guide, band-pass filters. Such filters represent a significant percentage of the overall system weight. High-capacity satellite communication systems usually distribute the signal power over the communication band of the system. In order to utilize the allocated frequency spectrum as efficiently as possible, guard bands should be kept very narrow and, hence, sharp cut-off filters are required.
At microwave frequencies, it is natural to utilize the tuned cavity of a wave-guide as one of the basic circuit elements in filter design. The dimensions of each cavity are determined by the desired center frequency of the band-pass filter. At the center frequency, the electrical length of each cavity must be equal to one-half or multiples of the guide wavelength for the particular mode under consideration.
A mode is the shape or configuration of a field (either electric or magnetic) in the cavity. In general, to produce the desired response from a filter, a cavity is configured to allow the passage of only a particular mode of the cavity's resonant frequency. The electromagnetic energy, restricted to this mode, emerges from the filter with the desired response.
Complex frequency responses can be realized with a minimum of additional cavities by using cavities designed to resonate in a plurality of modes, as shown by Atia et al., "New Types of Waveguide Bandpass Filters," Comsat Technical Review, Vol. 1, No. 1, Fall 1971, pp. 21-43, which is hereby incorporated by reference. For example, a dual-mode filter that initially resonates in a first mode has that first mode tuned or perturbed to create a second mode. The second mode differs from the first only in that the direction of its field is orthogonal to the field of the first mode. Through the use of such multiple-mode cavities, electromagnetic energy can be affected by a cavity's filter characteristic a plurality of times in one cavity rather than only once. As a result, the number of cavities necessary to produce the desired response can be reduced by one-half the number of corresponding single-mode sections required. The perturbation of the field in the first mode to produce a second orthogonal mode is generally called "coupling." Coupling invariably is caused by structural discontinuities in the cavity, such as screws positioned on its wall that perturb the field of the first mode. Coupling techniques are well known in the art. U.S. Pat. Nos. 4,410,865 and 4,734,665 provide examples of such techniques.
The resonant circuits of the microwave filters can be realized by the transverse electric (TE) or transverse magnetic (TM) modes which oscillate in resonance in the individual cavity resonators. The use of TE and TM modes to facilitate microwave communications in satellite systems is well known. U.S. Pat. Nos. 4,267,537, 4,489,293, 4,622,523, and 4,644,305, which are hereby incorporated by reference, each disclose the use in microwave filters used in satellite systems. Satellite systems often employ a number of directive antennas receiving signals at different frequencies. The signals received by the antennas are typically combined via microwave multiplexers. The multiplexer outputs the signals in a common channel of broader bandwidth, typically 500 MHz or more. Such multiplexer designs are well known in the art; U.S. Pat. Nos. 4,614,920 and 4,777,459 provide some examples.
FIG. 1 illustrates a conventional satellite communication repeater system. The output multiplexer section 5 consists of a set of high quality factor (Q) wave-guide cavities. In this particular example, the system is composed of five channels (shown in FIG. 2a), each designed to realize a six-pole, quasi-elliptic response. Each channel employs a narrow band-pass filter 21, 23, 25, 27 or 29 consisting of three dual-mode TE.sub.113 cavities. A series of low-pass filters 20, 22, 24, 26 and 28 are coupled to the input of each channel so as to suppress any potential higher order spurious transmission within the repeater.
In operation, an input multiplexer 2 (FIG. 1) divides or splits a band of signals received by receiving section 1 into a number of narrow-band frequency channels, e.g., 36 or 76 MHz. Separate high power amplifiers (within section 4) are used to amplify respective channel signals for input to the output multiplexer section 5. Each amplifier outputs signals to an associated low-pass filter (20, 22, 24, 26 or 28) which removes all high frequency noise signals from the channel, and outputs the filtered signal to an associated narrow band-pass filter 21, 23, 25, 27 or 29. Each narrow band-pass filter is designed to receive frequencies in the TE.sub.113 mode. Three dual-mode cavities are cascaded together to produce a wide-band response like that shown in FIG. 9a.
Each narrow band-pass filter output is coupled through a T-junction to a wave-guide manifold 36 (FIG. 2a). The output signals are summed together by the manifold to form a common output channel, and connected to an antenna for transmission to a ground station.
A major drawback of the repeater system shown in FIG. 1 is the use of a separate set of low-pass filters to separate the spurious noise from the input signal of each channel prior to the narrow band-pass filtering. The set of filters adds weight and components to the satellite system. Furthermore, the dual-mode wave-guide cavities have poor wide-band responses. That is, unwanted frequencies beyond the cavity's center frequency tend to appear, which causes the transmission response to become less predictable.
Thus, it is desirable to design a satellite repeater system with an output multiplexer filter which realizes a narrow bandpass response, but does not require additional components to be added to the system in order to produce a spurious-free wideband response.